There exists a continuing need for an arrangement effecting selective enabling/disabling of an existing functional block and an expansion functional block within a system (e.g., on a motherboard), while reducing the liklihood of malfunctioning or degrading (e.g., overloading) of a system clock. Considering a graphics controller functional block as an example, some resellers of computer systems desire to provide their customers with a selection of graphics capabilities, and thus desire to provide computer systems capable of being fitted with different graphics controllers. However, manufacturers generally desire to provide a generic graphics controller as standard equipment, while leaving it to the resellers to add a customized one at the time of sale of the system, if desired. In addition, some users of computer systems may at some time after the purchase decide to upgrade or change the graphics controller by inserting one in an expansion slot on the computer. Having two graphics controllers coupled to the computer system clock circuit at the same time might create an undesirable clock circuit load, and might cause edge rate degradation of the clock signals.
In the past, a dual in-line package (DIP) switch, a jumper, or other hardware on the motherboard was used to activate the add-on graphics controller, while disabling the built-in one. This is a cumbersome process employing tedious intervention by the user, in contrast to easier "plug-and-play" additions to the computer system.